In conventional steam turbine power plants, a temperature condition of steam is 600° C. or lower, and thus members of component parts to be exposed to high-temperature steam, for example turbine rotors, rotor blades, nozzles, and so on, are formed of ferritic heat resistant steels which excel in manufacturability and economical efficiency. On the other hand, carbon steels are employed as materials forming component parts not to be exposed to high-temperature steam, for example feed water heaters.
In late years, efficiencies of steam turbine power plants have been improved proactively for the reasons such as fuel saving and environmental preservation. For example, a steam turbine utilizing high-temperature steam at a temperature of about 600° C. (620° C. or lower) is in service. In such a steam turbine utilizing high-temperature steam, there exist quite a few parts that have characteristic requirements which the ferritic heat resistant steels are unable to satisfy. Accordingly, austenitic heat resistant steels or the like having more excellent high-temperature characteristics are used. However, using the austenitic heat resistant steels leads to increase in facility costs. Moreover, the austenitic heat resistant steels have low heat conductivity and a large linear expansion coefficient as compared to the ferritic heat resistant steels, and thus have a problem that heat stress occurs easily when a load changes during starting of a plant, stopping of a plant, or the like.
Further, what are called 700° C. class advanced ultra-supercritical (A-USC) power generating systems employing steam temperatures of 700° C. or higher are currently studied. When an entrance steam temperature of a steam turbine is 650° C. or higher, there occurs a part where the temperature of turbine extraction steam exceeds 580° C., which makes it necessary to use the heat resistant steels for a feed water heater performing heating with this extraction steam. However, delivering the turbine extraction steam at a temperature higher than 580° C. to the feed water heater is not favorable in view of heat stress, which is generated in proportion to the difference between a feed water temperature and an extraction steam temperature. For avoiding this, there is considered a cycle in which part of steam exhausted from a high-pressure turbine is supplied once to a back pressure extraction turbine to extract work, and extraction steam from the back pressure extraction turbine having decreased pressure and temperature is supplied to the feed water heater. Conventionally, this back pressure extraction turbine is connected directly to a feed water pump for driving the feed water pump.
As one cause of the global warming phenomenon, the greenhouse effect caused by carbon dioxide (CO2) has been pointed out. Accordingly, for example, methods for removing and collecting carbon dioxide in combustion exhaust gas by bringing the combustion exhaust gas into contact with absorbing liquid are studied actively, targeting at thermal power plants using a large amount of fossil fuel.
FIG. 9 is a diagram illustrating an example of a conventional carbon dioxide collecting system 300 which removes and collects carbon dioxide in combustion exhaust gas.
In the conventional carbon dioxide collecting system 300 illustrated in FIG. 9, for example, combustion exhaust gas exhausted by burning fossil fuel in a boiler is led to an absorbing tower 310 via a combustion exhaust gas supply port 311. Absorbing liquid 320 which absorbs carbon dioxide is supplied to an upper part of the absorbing tower 310, and this supplied absorbing liquid 320 is brought into gas-liquid contact with the delivered combustion exhaust gas and absorbs the carbon dioxide in the combustion exhaust gas.
The absorbing liquid 320 which absorbed the carbon dioxide is made to pass through a heat exchanger 340 from a lower part of the absorbing tower 310 by an absorbing liquid circulating pump 330, and is led to a recovery tower 350. In addition, the temperature of the absorbing liquid 320 which absorbed the carbon dioxide becomes higher than the temperature of the absorbing liquid 320 before absorbing the carbon dioxide due to thermal reaction heat by this absorption and sensible heat of the combustion exhaust gas.
On the other hand, the rest of the combustion exhaust gas from which the carbon dioxide is absorbed into the absorbing liquid 320 is emitted to the atmosphere from the upper part of the absorbing tower 310.
The absorbing liquid 320 led to the recovery tower 350 is heated in a reboiler 360 to discharge the absorbed carbon dioxide, and is recovered to be the absorbing liquid 320 capable of absorbing carbon dioxide again. The recovered absorbing liquid 320 is returned to the upper part of the absorbing tower 310 via the heat exchanger 340 by an absorbing liquid circulating pump 331.
On the other hand, the carbon dioxide discharged from the absorbing liquid 320 is led to a steam separator 370 via a cooler 341 to remove water therefrom, and is thereafter led to a carbon dioxide compressor 380 to be collected therein. Condensate separated in the steam separator 370 is led to the recovery tower 350. As the heat source for the reboiler 360, steam extracted from a steam turbine cycle in the thermal power plant or the like is mainly used. However, carbon dioxide gas raised to a high temperature in the process of compressing carbon dioxide can be used as well (see, for example, JP-B2 2809381 (Patent Registration) (hereinafter referred to as Reference 1) and JP-A 2004-323339 (KOKAI) (hereinafter referred to as Reference 2)).
For example, Reference 2 discloses a technique to deliver part of steam exhausted from a high-pressure turbine into a back pressure turbine for driving a carbon dioxide compressor, and deliver part of steam exhausted from an intermediate-pressure turbine into a back pressure turbine for driving auxiliary machines (for example, driving a feed water pump), thereby using steam exhausted from the steam turbines for heating in a carbon dioxide collecting system.
There are disclosed techniques related to a hybrid system which co-uses solar heat in a combined generation system (see, for example, JP-A 2008-39367 (KOKAI) (hereinafter referred to as Reference 3) and JP-A 2008-121483 (KOKAI) (hereinafter referred to as Reference 4)). In this hybrid system co-using solar heat, a heat exchanger absorbing solar heat is provided next to an exhaust heat recovery boiler of a combined generation cycle, thereby improving the fuel consumption of the entire generation cycle.